Method of preparing melts of zinc base alloys and improved flux therefor



United: States Patent 3,512,959 METHOD OF PREPARING MELTS OFZINC BASE-ALLOYS AND IMPROVED FLUX THEREFOR William F. Joseph, Rocky River, andJohn F. Wallace, Cleveland, Ohio, assignors to Rossborough SupplyCompany, Cleveland, Ohio, a corporation of Ohio No Drawing. Filed Sept.27, 1967, Ser. No. 671,114 Int. Cl. CZZb 19/30, 19/32 U.S. Cl. 75-87 7Claims ABSTRACT OF THE DISCLOSURE More eflicient recovery of usablemetal is obtained without objectionable smoke and fumes, iron pick up orloss of magnesium from the. alloy by adding to the dross layer on thesurface of a zinc base alloy bath a flux com: prising essentially 20 to60 percent by weight of magnesium chloride (calculated as anhydrous) andat least 40 percent by weight total of sodium and potassium chlorides,the temperature to which the flux is heated being above the meltingpoint of the flux. The sodium chloride may be present in amounts of 0 to45 percent by weight and the potassium chloride may be present inamounts of 0 to 60 percent by weight although. a superior [flux resultswhen sodium and potassium chlorides are each present in amounts of 20 to40 percent of the total. When the zinc alloy contains aluminum, theimproved flux preferably contains a small amount, up to 20 or 25%, ofthe total of said chlorides of an inorganic fluoride selected fromsodium and potassium cryolites and silicofluorides and mixtures of oneor more thereof.

The present invention relates'to a method of fluxing,

melting and maintaining zinc alloys in a form suitable for die castingor other melting of zinc and to an improved flux therefor. In themelting of zinc base alloys for die casting or other purposes, the metalis heated to a temperature of 780 F. or above, usually at 850 to 1000 F.During the melting operation, parts of the melted metal Within the drosson the bath surface are often prevented from coalescing with other parts'because of the presence of oxides. The result is that a considerableamount of metal is trapped in the dross layer over the molten metal.This dross layer is made up primarily of uncoalesced molten particles ofthe alloy, zinc oxide and an iron-aluminum compound.

In order to facilitate coalescing of major portions of the molten metalso that it is returned to the melt, it is customary to add to the drosslayer on the surface of the metal during initial and subsequentmeltings, substantial quantities of a flux. The fluxes used heretoforehave facilitated coalescence of a major portion or most of the moltenmetal particles in the dross layer so that the metal is returned to thebath.

The fluxes used heretofore and considered to be most eflicient containedlarge amounts of zinc chloride and ammonium chloride both in mechanicaland chemical combination. The zinc chloride-ammonium chloride fluxescause a strong exothermic reaction in the dross. layer and have beenconsidered to do an excellent job of releasing entrapped metal in thedross layer. They produce a desirable dry, powdery dross. The strongexothermic reaction is, however, not desirable for it raises thetemperature of the melt above that desired and makes melt temperaturehard to control. Both the zinc chloride and the ammonium chloride causea large amount of smoke (visible and highly objectionable vapors) to beevolved. These vapors are toxic, tend to cause illness, and constitute ahealth hazard. Authorities concerned with air pollu- Patented May 19,1970 tion have objected to the discharge of such smoke into theatmosphere. Compliance with the smoke abatement requirementsnecessitates the installation and operation of exepensive equipment bycompanies using the prior fluxes.

Zinc die casting alloys contain .02% to .08% of magnesium to increasetolerance of lead, cadmium and tin impurities, and about 4% of aluminumto yield eutectic type alloys. The reactions occurring when fluxescontaining zinc chloride-ammonium chloride are used cause a loss ofmagnesium from the alloy. A decrease of the magnesium content results inthe inter granular corrosion of the castings made of such alloy. Tocompensate for such loss it has been a practice to add some magnesiumchloride to the flux.

It is a prime object of the present invention to provide a flux whichpermits melting of zinc alloys without obnoxious smoke or highly toxicfumes.

It is another object of the present invention to provide fluxes for zincalloys which produce a finely powdered dross with a higher recovery ofmetal from such dross than has been obtainable with the prior art.

It is still another object of the invention to provide more efficientfluxes for zinc 'base alloy baths that produce a powdery dross, increasethe recovery of metal therefrom, produce no appreciable exothermicreaction when used, do not cause detrimental consumption or loss ofmagnesium from the zinc alloy bath, and reduce aluminum loss from thealloy because of the absence of said exothermic reaction.

It is still another object. of this invention to provide a method oftreating zinc base alloys during the melting, alloying or castingthereof, which method increases the proportion of usable metal over thatbefore obtainable, provides a dross that is powdery and easily removedfrom the metal and does not cause objectionable smoke.

It is another object of the present invention to provide a method ofrecovering more efiiciently usable metal from the skimming of zinc basealloy baths. Such skimmings being the dross layer which has been removedfrom the surface of a zinc alloy bath. These skimmings are treatedseparately with fluxes.

We have found in accordance with the present invention that the aboveand other objects are obtained by treating the dross upon zinc basealloy baths, with a flux comprising essentially magnesium chloride andat least one chloride selected from sodium and potassium chlorides. Ournew flux is preferably obtained by mixing certain proportions ofmagnesium chloride with both sodium chloride and potassium chloride and,preferably, in cases where aluminum is in the alloy, some inorganicternary fluoride compound selected from one or more of cryolite (Na AlFpotassium cryolite (K AlF sodium silicofluoride (Na SiF and potassiumsilicofluoride (K SiF The silicofluorides and cryolites can provide amild exothermic reaction when the flux is added to the surface of a zincalloy bath.

Our new flux should have little if any ,zinc chloride and ammoniumchloride as even a comparatively small amount of these causes smoking.However, up to 5% or so of each of these materials based on the totalweight of MgCl KCl and NaCl present can be tolerated in manyapplications.

The magnesium chloride (MgCl considered'asanhydrous, should constituteat least 20% by Weight of the 1 total of the magnesium, sodium andpotassium chlorides.

When more than 60% of MgCl is present, the dross becomes wet and is hardto manage. When the amount of magnesium chloride is less than 20%, themelting point of the flux becomes inordinately high. The preferredamount of magnesium chloride is 25% to 60% of the total of themagnesium, sodium and potassium chlorides in the flux. The optimumamount depends to some extent on the melting temperature or operationaltemperature of the melting unit.

The exact function of the sodium and potassium chlorides is not fullyunderstood, but they cooperate with the magnesium in some manner toproduce the unique properties in the flux. When the total of these twomaterials is less than 40% of the total of the MgCl and alkali metalchlorides the dross becomes wet.

We have found that to be elfective the flux must melt at or below theoperating temperature of the zinc alloy bath. Operational fluxesinclude, therefore, those mixtures of magnesium chloride with potassiumand/or with sodium chloride which melt at or below the operatingtemperature of the zinc alloy bath, which is usually from about 780 to1050 F.

The sodium chloride should constitute about to about 40% of the weightof the total of alkali chlorides and MgCl present, and about 20% toabout 40% is preferred.

The potassium chloride should be present in a maximum amount of 60% ofthe weight of the MgCl plus total alkali chlorides present. When atleast 40% of these chlorides is made up of sodium, then the potassiumchloride may be omitted and still a usable flux obtained, provided theoperating temperature of the zinc alloy bath is above the melting pointof the flux. Both sodium and potassium chlorides are preferably present,however, and the potassium chloride preferably should be present inamounts of about 19% to about 40% of the total of MgCl and alkali metalchlorides present.

Our new fluxes when applied to the surface of a zinc alloy bath are noteven mildly exothermic unless the aforesaid fluorides are present. Theapplication of our fluxes to the surface of a zinc alloy bath serves torelease a substantial portion of the free metallic zinc particlesentrapped in the surface covering or dross. Subsequently the drossremaining becomes dry and powdery and is easily removed from the surfaceof the bath. It has been found that these skimmings have high heatinsulating characteristics such that the remaining spheroids of moltenZinc which were not released when the flux was applied to the surface ofthe bath can be agglomerated and salvaged apart from the melting bath.By simple stirring of the skimmings, this free metal accumulates to suchan extent that the metallic content thereof is de creased to about 0.5%,or even less. This compares with 0.8% for the best obtainable practiceheretofore followed and with the 2% to 3% resulting in the use ofordinary practices. We have found no loss of aluminum and no pick up ofiron in melts prepared and maintained when using our new fluxes.

The magnesium chloride used need not be anhydrous if care in use of ournew flux is practiced. The above percentages are, however, based onanhydrous MgCl Hydrated magnesium chloride (MgCl '6H O) is considerablyless expensive to use than the anhydrous based on magnesium chloridecontent. When hydrated magnesium chloride is used the flux must be addedgradually to the surface of the alloy bath to permit gradualvaporization of the water. One cannot plunge the flux into a moltenbath. If such a procedure is used anhydrous MgCl should be used. We havefound that the effectiveness of our new flux is increased greatly whenit is stirred or worked in the dross on the surface of the molten metal.

The following examples, in which percentages are by weight, illustratethe invention.

EXAMPLE 1 Percent MgCl '6H O NaCl 27.3 KCl 27.4 K A1Fs or Na3AlF 3.0

The above ingredients are thoroughly mixed and packaged in a closedcontainer.

EXAMPLE 2 1,000 pounds of a zinc die casting alloy containing about0.05% of magnesium and 4% of aluminum is placed in a ferrous melting potof a melting furnace along with 1.8 to 2 pounds of the flux ofExample 1. The alloy is heated to 850 F. The flux is worked or stirredinto the dross on the surface of the molten metal to increase theeffectiveness. No noticeable fumes or smoke is evolved and the drossbecomes powdery. Upon removal from the furnace and subsequent stirringthe metallic content thereof was found to be less than 0.5% of theweight thereof, and the magnesium and aluminum content of the meltremain unchanged.

EXAMPLE 3 To the ingredients of Example 1 is added 10% (by weight of theoriginal mixture) of Na SiF The resultant flux is somewhat superior tothat of Example 1 when used in Example 2. The coalescence of the metalis faster.

EXAMPLE 4 The flux of Example 3 is applied on the surface of a suitablezinc alloy melt, as described in Example 2.. The alloy is heated to atemperature of between 780 and 1,050 F. No visible fumes are evolvedfrom the flux and the magnesium and aluminum content of the metal remainstable.

EXAMPLE 5 Three separate fluxes based on MgCl -6H O were prepared bymixing together the ingredients shown in the following table:

Flux (percent) A B o 42 38 3s 2s 26 26 2s 25 25 2 1 1 0 10 0 0 0 10EXAMPLE 6 1,000 grams of skimmings, resulting from melting and skimmingthe dross from zinc die cast gates and sprues were melted in a graphitepot at 1000 F. To these molten skimmings were added 30 grams of Flux Babove by slowly mixing the flux into the skimmings. A powdery dross wasobtained and 935 grams of free metal alloy were recovered from theskimmings, representing a yield of 93.5%.

EXAMPLE 7 The procedure of Example 6 Was repeated except that Flux C ofExample 5 was substituted for Flux B. The fre metal alloy recovered was92.8% of the weight of the skimming.

EXAMPLE 8 The procedure of Example 6 was repeated except that Flux A ofExample 5 was substituted for Flux B. 910 grams of free metal wasrecovered from the skimmings.

EXAMPLE 9 To 150 pounds of skimmings from a melt of zinc die cast scrapwas added 4 pounds of Flux A of Example 5. The mixture was also heatedto 1000 F. in a graphite pot while stirring and working the flux intothe molten skimmings. 135 pounds of free metal alloy were recov-: eredfor a yield of based on the weight of the skimmmg.

In making melts the new fluxes are generally used in amounts of about.15 to .25 of the weight of the zinc base alloys melted, although aslittle as .05% is effective andup to 1.0% or more may'be required. Inrecovering metal from the skimmings front zinc base alloys, tluxes inamounts of 1 to 5%, based on the weight of the skimniings, are usuallyused, although as little as 5% gives considerable recovery, and even ormore may be used. About 2 to 4% is optimum from an economic standpoint.

When the flux is mixed slowly or stirred into theskimmings, heatsufficient to melt the flux must be used and the temperature may varyfrom 780 F. to 1200 F. Usually the heating is in the presence of moltenmetal and about 780 F. to 1050 F. is then preferred, although highertemperatures. even 1200 1 1, may he used.

It is to he understood that in accordanccwith the provisions of thepatent laws, various modifications can he made in the method,procedures, compositions and products of the present invention withoutdeparting from the spirit of the same.

We claim:

1. A process of recovering usable metal from the skimmings resultingfrom melting zinc base alloys which comprises heating the said skimmingsin intimate contact with a mixture of MgCl and at least one of the groupconsisting of NaCl and KCl, said MgCl being present in amounts of to 60percent by weight and said group members being present in a total of atleast 40 percent by weight, 0 to 40 percent of NaCl, and 0 to 60 percentof KCl being present, said heating being at a temperature at least equalto the melting. point of said mixture.

2. The process according to claim 1 wherein both NaCl and KO arepresentin said mixture in amounts of 20 to 40 percent by weight I Y 3.The process according to claim 2 wherein 1 to (based on weight of saidmixture) of an inorganic fluoride compound; selected from the groupconsisting of sodium and potassium 'cryolites, sodium and potassiumsilicofluorides and mixtures thereof, is also present in admixture withsaid mixture of MgC1 KCl and NaCl.

4. The process of claim 1 wherein said mixture is free from more than 5%by weight each of zinc and ammonium chlorides.

5. In a process of treating melts of a zinc base alloy wherein the metalis heated to a temperature of between 780 and 1050 F., the steps whichcomprise applying a flux comprising 20 to 60% by weight of magnesiumchloride (calculated as anhydrous) and at least total of sodium andpotassium chlorides, the sodium chloride being present in amounts of 0to 40% and the potassium chloride'being present in amounts of 0 to anyzinc chloride and ammonium chloride present being each less than 5% ofthe total of said magnesium, sodium and potassium chlorides to thesurface of the molten metal, stirring the flux on the said surface andstirring the resultant powdery dross to agglomerate spheroids of moltenmetal and return them to the metal bath.

6. In a process of treating vmelts of a zinc base alloy wherein themetal is heated to a temperature of between 780" and 1050" the stepswhich comprise applying a flux comprising a mixture of MgCl and at leastone of r the group consisting of NaCl and KCI, said MgCl being presentin amounts of 20 to 60 percent by weight and said group members beingpresent in a total of at least 40 percent by weight, 0 to 40 percent ofNaCl, and 0 to 60 percent of KCl being present to the surface of thebath; stirring the flux on said surface and stirring the resulting,

powdery dross to agglomerate spheroids of molten metal and return themto the metal bath. 7. The process of claim 6 wherein the powdery drossis removed from the metallic bath and subsequently stirred to effectagglomerationof the metal spheroids contained therein.

References Cited UNITED STATES PATENTS HENRY'W. TARRING 11, PrimaryExaminer US. Cl. X.R. -65, 93, 94

